An electronic transistor is the basic building block of today's computer logic circuits. Transistors use an electric current or field voltage to amplify another electric current or field voltage, creating electronic pulses that can represent the ones and zeros of binary computing. In the optical realm, transistors can function beyond optical logic operations in optical chips or optical computing, into areas of telecommunication networks. Optical transistor technology could be utilized for optic fiber communication, optical switching and routing, and wavelength conversion.
Telecommunication networks have expanded considerably over the last several decades. Although most calls, whether carrying voice (telephone calls) or data, have long used standard telephone lines, which have a low bit rate, the formidable expansion of the Internet and all other data networks, whether in the public or private sector, since the middle eighties has led to an enormous demand for bandwidth. To face up to this exponential increase in the quantity of information to be transported, and which relates to all types of media, i.e. as much to voice as to data, such as electronic mail (E-mail), text and picture file transfer, video distribution and, most importantly, the massive use of the Internet and the World Wide Web (WWW), new technologies have had to be developed, as transmission over electrical media (metal lines, copper) has proved to be too limited in performance over long distances.
At least insofar as the core of these networks is concerned, transmission is now mostly via optical fibers at very high bit rates. The rate of exchange of data, or information bits, is routinely measured in gigabits per second. This means that one billion bits can be exchanged every second over a 1 Gbit/s line. In practice, international standards exist to standardize transmission and to ensure the interworking of equipment. The most widespread of these standards is the SONET (Synchronous Optical NETwork) standard. The SONET standard is primarily a North American standard, and its European counterpart is the SDH (Synchronous Digital Hierarchy) standard. These standards are for the most part mutually compatible and standardize transmission speeds of 2.48 Gbit/s (SONET OC-48), 10 Gbit/s (SONET OC-192) and 40 Gbit/s (SONET OC-768).
Although communication equipment now communicates via a network of optical fibers, which can be very extensive and cover a city or a country, and can include intercontinental transmission, and carries pulses of light generally obtained from a coherent light emitter (laser), it remains the case that the communication equipment itself is still essentially based on electrical technologies and the peripheral circuits that constitute the equipment must be capable of being interfaced efficiently and at low cost to the devices sending and receiving light signals interfaced to the optical fibers.
In its simplest form, binary signals conveying the information bits referred to above are simply transmitted by modulating at two levels the light emitter, usually a laser. Thus the optical signal is generated at two power levels and the laser is switched from a level at which it emits a sufficient quantity of light to be received by the optical receiver situated at the other end of the fiber to a level at which it does not emit any or much light, in which state it must be considered to be turned off. The receiver is thus in a position to discriminate the two levels corresponding to an information bit (a ‘1’ generally corresponding to the state in which the laser is emitting light, although the opposite convention is obviously equally feasible). If the emitter continues to emit between two consecutive ‘1’ and returns to the off state only to transmit a ‘0’, the modulation mode is known as non return to zero (NRZ) modulation. It is cheap and well suited to the mode of operation of lasers, which are turned on or off to transmit each bit of information.
It is extremely difficult, however, to create transistors with light beams. There exist several approaches to develop optical transistors, most of which are based on nonlinear optical effects or local plasma effects. Even through these approaches offer very fast operation, they have not been widely applied because special materials and very high light energies are required.
Most of efforts on developing optical transistors are concentrated on utilizing nonlinear optical effects or local plasmon effects. For example, one of invented optical transistor is based on cross-phase-modulation (XPM) induced polarization change from the third-order nonlinear susceptibility χ3 in optical materials. Another research performed by a Japanese group utilizes local plasmon, which are waves of electrons on the surface of a metal, store energy. Even though they can offer extremely fast speed to perform optical logic operation or signal amplification, these technologies have not been widely applied. One of the major reasons is limited material choice such as high nonlinear optical materials. Another drawback is high threshold, which severely restricts application areas. Moreover, many existing optical transistor technologies do not have flexibility, which can be used for many purposes. Furthermore, it is difficult for some of them to be integrated into current manufacture processes. Finally, some technologies cannot be used to build optical transistor arrays.
Therefore, a need exists for an improved optical transistor technology capable of overcoming the aforementioned limitations.